1. Field of Invention
The present invention relates to a direct current offset canceling circuit, particularly to a direct current offset canceling circuit of trans-impedance amplifier (TIA) and an automatic gain control trans-impedance amplifier thereof.
2. Related Art
The main function of a trans-impedance amplifier is to convert a photocurrent signal into a voltage signal and amplify it, and the input photocurrent signal includes a direct current (DC) component and an alternating current (AC) component, wherein the DC component part will influence the operation of the trans-impedance amplifier, therefore, it is necessary to design a circuit for canceling the DC component such that the trans-impedance amplifier can be operated normally. The circuit architecture of a general trans-impedance amplifier is shown in FIG. 1, which includes a trans-impedance amplifier 10, a reference voltage unit 40 and a voltage output amplifier 50. The trans-impedance amplifier 10 further includes an operational amplifier 10a and a resistor 10b, wherein the both ends of the resistor 10b are connected respectively to the input and output of the operational amplifier 10a. And the reference voltage unit 40 further includes an operational amplifier 40a and a resistor 40b, wherein the both ends of the resistor 40b are connected respectively to the input and output of the operational amplifier 40a. When the input photocurrent signal includes a DC component, the DC component part will influence the operation of the trans-impedance amplifier 10.
In recent trans-impedance amplifier direct current offset canceling circuit techniques, the related arts of U.S. Pat. No. 6,774,728 and No. 6,833,762 disclose respectively a trans-impedance amplifier DC offset canceling circuit and a trans-impedance amplifier DC offset canceling circuit having an automatic gain control, which mainly for retrieving signals in the input and output of the trans-impedance amplifier to obtain its DC offset amount and eliminate it correspondingly, to solve the problem of the DC component influence to the trans-impedance amplifier operation.
Therefore, one of the tasks to be addressed by the researchers is how to provide a DC offset canceling circuit of trans-impedance amplifier.